Anthony James Howard

The relation between porosity, microstructure and strength, and the approach to advanced cement-based materials

A theory is formulated to connect the strength of cement paste with its porosity. The theory shows that bending strength is largely dictated by the length of the largest pores, as in the Griffith (1920) model, but there is also an influence of the volume of porosity, which affects toughness through changing elastic modulus and fracture energy. Verification of this theory was achieved by observing the large pores in cement, and then relating bending strength to the measured defect length, modulus and fracture energy. The argument was proved by developing processes to remove the large pores from cement pastes, thereby raising the bending strength to 70 MPa. Further removal of colloidal pores gave a bending strength of 150 MPa and compression strength up to 300 MPa with improved toughness. Re-introduction of controlled pores into these macro-defect-free (mdf) cements allowed Feret’s law (1897) to be explained.

On the hydration of Portland cement

When Portland cement is contacted with water, calcium ion is rapidly leached from the solid to form calcium hydroxide solution but only traces of silica are found in the aqueous phase. It is proposed that the hydrated, calcium-depleted surface of grains consists of low molecular mass silicic acids and that these interact with dissolved hydroxylated calcium species (principally Ca(OH)2) to produce a semi-permeable membrane of ‘ calcium silicate hydrate ’ at the hydrated grain surface. Osmotic pressure within this membrane causes its rupture and hence the growth of excrescences from the grain as the contents are extruded into the surrounding calcium hydroxide solution. The interstitial solid material is best regarded as a coagulum resulting from the combination of low molecular mass silicate anions with dissolved calcium hydroxide. It is proposed that the low tensile strength of Portland cement paste results from microstructural features consequent upon this mechanism of hydration and setting.

Advanced materials from hydraulic cements

Hydraulic cements are energy-cheap relative to other common materials and this was the driving force that led to the development of macrodefect-free (MDF) cements in 1981. In this paper the scientific principles of such materials in terms of porosity, particle packing and rheology are demonstrated and the use of a range of different cement types will be reviewed. The role of the polymeric rheological aid is emphasized and distinguished from that of so-called super-plasticizers used with conventional cements. The distinction between MDF cements and polymer concretes of various types is highlighted and the similarity of MDF cements and chemically bonded ceramics (CBCS) when the polymer is removed is noted. Various properties of MDF cements are given with emphasis on their high degree of mechanical reliability. These properties place MDF cements in a unique position in the materials field, a position that makes the replacement of hitherto more advanced engineering materials, such as metals, polymers and ceramics, a viable proposition. Areas of use are discussed.